chore: add landmark elements to dev app (angular#14436)

Fixes the dev app not having landmark elements. This removes some of the noise when doing accessibility checks with aXe.

Author: crisbeto

src/dev-app/dev-app.html

@@ -1,5 +1,5 @@
 <mat-sidenav-container class="demo-container">
-  <mat-sidenav #start>
+  <mat-sidenav #start role="navigation">
     <mat-nav-list>
       <a *ngFor="let navItem of navItems"
          mat-list-item
@@ -22,7 +22,7 @@
     </mat-nav-list>
     <button mat-button tabindex="-1" (click)="start.close()">CLOSE</button>
   </mat-sidenav>
-  <div>
+  <main>
     <mat-toolbar color="primary">
       <button mat-icon-button (click)="start.open('mouse')">
         <mat-icon>menu</mat-icon>
@@ -45,5 +45,5 @@ <h1>Angular Material Demos</h1>
     <div #root="dir" dir="ltr" class="demo-content">
       <router-outlet></router-outlet>
     </div>
-  </div>
+  </main>
 </mat-sidenav-container>